Image processor

ABSTRACT

In an image possessor, two kinds of color differences in the orthogonal coordinate axes in the hue plane in the color space are separated from the digital data of three primary colors, and the separate color signals are synthesized to generate the chroma signal. When the digital data of the three primary colors are converted to the data of the reproduction colors with use of the under color remove amount and the black paint amount, an under color remove amount is subtracted from the digital data of the three primary colors, while a black paint amount which is the black data obtained from the digital data is generated to print with black toners. The under color remove amount and the black paint amount are increased with increase in the chroma signal according to the color data of the three primary colors and the chroma signal at the same pixel position. The edge emphasis is performed not on the image data of the three primary colors in the subtractive color system, but on the image data of the black color, in an edge image portion of achromatic color. Further, in the edge emphasis, a first change in the value signal and a second change in the image data for image reproduction are detected, and the two changes are synthesized. Then, the edge emphasis is conducted on the image data for printing or on the image data for color reproduction according to the synthesized change.

BACKGROUND OF THE INVENTION

1 Field of the Invention

The present invention relates to an image processor which processesdigital image data for image reproduction.

2. Description of the Related Art

In a printer or the like wherein an image of full color is reproduced,digital image data R, G, B of three primary colors of red, green andblue of the additive color system which have been read by scanning adocument are converted for image reproduction to data C, M, Y of threecolors of cyan, magenta and yellow for color reproduction (ofsubtractive color system).

In the data processing of a full color image to convert the digital dataof red, green and blue of the three primary colors to the data of thecolors for image reproduction, it is demanded to reconcile the purity ofblack and the brightness of chromatic colors.

In the reproduction of black in a full color image, when black isreproduced by overlaying cyan, magenta and yellow, it is difficult toreproduce pure black due to the effects of the spectral characteristicsof each toner. Then, the reproducibility of black is improved by usingthe subtractive mixing of the reproduction color data C, M and Y andblack painting of black data K. That is, the reproduction color data C,M and Y are subtracted by a certain amount (under color remove amount),while the black data K is obtained from the reproduction color data C, Mand Y to reproduce black by using black toner. However, in this method,though the purity of black is improved with increasing the degree ofblack painting, whereas the brightness of chromatic colors isdeteriorated.

As explained above, the reproducibility of black and the improvement ofthe brightness of full color are not compatible as to the under colorremove amount for chromatic colors and the amount of black paint forblack. Therefore, it is necessary to control optimally the subtractionamount for chromatic colors and the amount of black paint for black.

It is better to emphasize an edge for an image such as a character. Theread data is converted to the density data in order to adjust to thesense of naked eyes. When the edge emphasis is conducted on the dataafter the density conversion of the read data, the following problemsarise: As to an edge portion of a character or a narrow line, the datachanges more smoothly than before the density conversion, and sufficientedge emphasis is not performed. Further, the data change at the highdensity side is large so that an edge is easily detected at the highdensity side and even image noises are emphasized. This is ascribable tothat the slope of the -log characteristic is small at the low densityside and large at the high density side. As to a full color image, thehue changes if the edge emphasis is performed on the data after thecolor correction.

SUMMARY OF THE DISCLOSURE

It is an object of the present invention to provide an image processorwherein the under color remove amount and the black paint amount can becontrolled optimally in order to reconcile the reproducibility of blackand the improvement of the chromaticity of the chromatic colors.

It is another object of the present invention to provide an imageprocessor wherein the edge emphasis can be performed optimally.

In one aspect of the present invention, the digital image data R, G, Bof three primary colors are converted to the value V and the chroma W.That is, two kinds of color differences B-V, R-V in the orthogonalcoordinate axes in the hue plane in the color space are obtained fromthe digital data R, G, B, to be synthesized to generate the chromasignal. When the digital data R, G, B are converted to the data of thereproduction colors, an under color remove amount is subtracted from thedigital data of the three primary colors, while a black paint amountwhich is the black data obtained from the digital data is generated toprint with black toners. The under color remove amount and the blackpaint amount are increased with increase in the chroma signal accordingto the color data of the three primary colors and the chroma signal atthe same pixel position. Because the data of the reproduction colors andthe black are determined according to the under color remove amount, theblack paint amount, the digital data and the chroma, the erroneousdecision decreases and the range of the control is extended to 100%.Thus, the under color remove amount and the black paint amount can bedetermined optimally.

In another aspect of the present invention, the edge emphasis isperformed not on the image data of the three primary colors in thesubtractive color system, but on the image data of the black color, inan edge image portion of achromatic color.

Further, in the edge emphasis, a first change in the value signal and asecond change in the image data for image reproduction are detected, andthe two changes are synthesized. Then, the edge emphasis is conducted onthe image data for printing or on the image data for color reproductionaccording to the synthesized change.

It is an advantage of the present invention that the under color removeamount and the black paint amount can be determined optimally in thereproduction of a full color image.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings, and in which:

FIG. 1 is a sectional view of a full-color copying machine;

FIG. 2 is a block diagram of a part of the control system of the copyingmachine;

FIG. 3 is a block diagram of the other part of the control system of thecopying machine;

FIG. 4 is a perspective view of a reading device;

FIG. 5 is a block diagram of an image signal processor;

FIG. 6 is a circuit diagram of a timing controller;

FIG. 7 is a circuit diagram of an HVC line memory interface;

FIG. 8 is a circuit diagram of an RGB line memory interface;

FIG. 9 is a diagram of the peripheral of CPU;

FIG. 10 is a circuit diagram of a shading correction section;

FIG. 11 is a diagram of a black correction block of the shadingcorrection section;

FIG. 12 is a part of a timing chart at positions (a)-(i);

FIG. 13 is another part of the timing chart at the positions (a)-(i);

FIG. 14 is a part of a flowchart of shading correction;

FIG. 15 is another part of the flowchart of shading correction;

FIG. 16 is a diagram of the sequence of shading correction;

FIG. 17 is a plan view of a part of a CCD sensor;

FIG. 18(a) is a diagram of an image of black and white, FIG. 18(b) is agraph of color resolution data of the image and FIG. 18(c) is a graph ofa read data of the image after position correction;

FIG. 19 is a diagram of the Munsell color system;

FIG. 20 is a block diagram of an HVC converter;

FIG. 21 is a block diagram of a chroma extraction circuit;

FIG. 22 is a block diagram of a hue extraction circuit;

FIG. 23 is a circuit diagram of a density converter;

FIG. 24 is a schematic block diagram of a region discrimination sectionin the image signal processor;

FIG. 25 is a schematic block diagram of a region discrimination sectionfor MTF automatic control in the monochromatic mode;

FIG. 26 is a part of a circuit diagram of a region discriminationsection;

FIG. 27 is another part of the circuit diagram of a regiondiscrimination section;

FIG. 28 is a graph of the BK level (D) plotted against a minimum value(MIN);

FIG. 29 is a diagram of primary differential filters;

FIG. 30 is a diagram of a smoothing filter;

FIG. 31 is a graph of the conversion of a UCR/BP control table;

FIG. 32 is a graph of a change of an emphasis amount of an MTF controltable;

FIG. 33 is a diagram of read data of an image shown above and theprocessing thereof;

FIG. 34 is a graph of black data;

FIG. 35 is a graph of the characteristic of a green filter;

FIG. 36 is a graph of the characteristic of magenta toners;

FIG. 37 is a graph of read data of a gray scale;

FIG. 38 is a graph of a part of the result of automatic UCR/BPprocessing of the data shown in FIG. 37;

FIG. 39 is a graph of read data of an image of black and white with useof each element of a CCD sensor (the abscissa denotes the address);

FIG. 40 is a graph of density converted from the data of FIG. 39;

FIG. 41 is a graph of HVC conversion of the data of FIG. 39;

FIG. 42 is a graph of the edge of value and of chroma of the data ofFIG. 41;

FIG. 43 is a diagram of the result of automatic UCR/BP processing of thedata of FIG. 39;

FIG. 44 is a graph of read data of an image of red and white with use ofeach element of a CCD sensor (the abscissa denotes the address);

FIG. 45 is a graph of density converted from the data of FIG. 44;

FIG. 46 is a graph of HVC conversion of the data of FIG. 44;

FIG. 47 is a graph of the edge of value and of chroma of the data ofFIG. 46;

FIG. 48 is a diagram of the result of automatic UCR/BP processing of thedata of FIG. 44;

FIG. 49 is a diagram of an example of the data processing for borderingin a bordering mode;

FIG. 50 is a part of a circuit diagram of color correction section;

FIG. 51 is another part of the circuit diagram of color correctionsection;

FIG. 52 is a circuit diagram of a register;

FIG. 53 is a circuit diagram of an MTF correction section;

FIG. 54 is a circuit diagram of a register and the peripheral thereof;

FIG. 55 is a diagram of a smoothing filter;

FIG. 56 is a diagram of a Laplacian filter;

FIG. 57 is a diagram of the processing with use of a Laplacian table;

FIG. 58 is a diagram of edge emphasis with use of a Laplacian filter;

FIG. 59 is a graph of read data of R, G and B;

FIG. 60 is a graph of the result of automatic UCR/BP processing of thedata of FIG. 59;

FIG. 61 is a graph of the result of automatic MTF processing of the dataof FIG. 59;

FIG. 62 is a graph of the result of color blur correction of the data ofFIG. 59;

FIG. 63 is a diagram of contour extraction in contour extraction mode;

FIG. 64 is a graph of value data and its primary derivative of adocument of 1 line pair/mm;

FIG. 65 is a graph of density data and its primary derivative of thedata of FIG. 64;

FIG. 66 is a graph of the result of the automatic MTF correction of thedata of FIG. 64;

FIG. 67 is a graph of the result of smoothing (comparison example) ofthe data of FIG. 64;

FIG. 68 is a graph of the result of edge emphasis (comparison example)of the data of FIG. 64;

FIG. 69 is a graph of density data of a dot image;

FIG. 70 is a graph of the result of the automatic MTF correction of thedata of FIG. 69;

FIG. 71 is a graph of the result of edge emphasis (comparison example)of the data of FIG. 69;

FIG. 72 is a graph of the result of smoothing (comparison example) ofthe data of FIG. 69;

FIG. 73 is a graph of value data and its primary derivative of adocument of black lines of 1 mm width;

FIG. 74 is a graph of density data and its secondary derivative of thedata of FIG. 73;

FIG. 75 is a graph of the result of the automatic MTF correction of thedata of FIG. 74;

FIG. 76 is a graph of the result of smoothing (comparison example) ofthe data of FIG. 74; and

FIG. 77 is a graph of the result of edge emphasis (comparison example)of the data of FIG. 74.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A digital color copying machine of an embodiment of the presentinvention will be explained below in the following order:

(a) structure of digital color copying machine

(b) image data processor

(c) shading correction

(d) HVC conversion

(d-1) R, G, B read position correction

(d-2) separation of chroma and hue

(d-3) circuit of HVC conversion section

(e) density conversion

(f) region discrimination

(f-1) circuit of region discrimination section

(f-2) automatic control of under color remove/black painting (UCR/BP)

(f-3) automatic control of edge emphasis

(f-4) decision of achromatic edge portion

(g) color correction

(g-1) automatic UCR/BP processing

(g-2) masking processing

(g-3) example of automatic UCR/BP processing

(g-4) monochromatic color mode and color change mode

(g-5) bordering processing

(g-6) circuit of color correction section

(h) MTF correction

(h-1) circuit of MTF correction section

(h-2) smoothing

(h-3) edge emphasis

(h-4) edge emphasis and sharpness mode

(h-5) color blur correction

(h-6) contour extraction mode

(h-7) examples of automatic MTF correction

(a) structure of digital color copying machine

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the views, FIG. 1 showsa schematic structure of a digital color copying machine which consistsmainly of an image reader 100 for reading a document image and a mainbody 200 for reproducing the document image.

In FIG. 1, a scanner includes an exposure lamp 12, a rod lens array 13to collect reflection light from a document put on a platen 15 and acontact type CCD color image sensor 14 to convert the collected light toan electric signal. The scanner 10 is driven by a motor 11 to move inthe direction (subscan direction) of the arrow shown in FIG. 1. Theoptical image of the document illuminated by the exposure lamp 12 isconverted by the image sensor 14 into a multi-level electric signal ofred (R), green (G) and blue (B). The electric signal is converted by animage signal processor 20 to gradation data of yellow (Y), magenta (M),cyan (C) or black (K). Then, a print head 31 performs the gammacorrection of the gradation data and a dither processing if necessary,and it converts the corrected data to a digital drive signal to drive alaser diode 221 (not shown) in the print head 31.

A laser beam emitted from the laser diode 221 according to the gradationdata exposes a photoconductor drum 41 driven to be rotated, via areflection mirror 37 as shown with a dot and dash line. Thus, an imageof the document is formed on the photoconductor of the drum 41. Thephotoconductor drum 41 has been illuminated by an eraser lamp 42 and hasbeen sensitized uniformly by a sensitizing charger 43 for each copybefore the exposure. When the exposure is performed onto thephotoconductor in the uniformly charged state, an electrostatic latentimage is formed on the photoconductor drum 41. Then, one of developers45a-45d of yellow, magenta, cyan and black toners is selected to developthe latent image. The developed image is transferred by a transfercharger 46 to a paper wound on a transfer drum 51.

The above-mentioned printing process is repeated four times for yellow,magenta, cyan and black. At this time, the scanner 10 repeats thescanning in synchronization with the motion of the photoconductor drum41 and the transfer drum 51. Then, the paper is isolated from thetransfer drum 51 with the operation of an isolation claw 47, the imageis fixed by a fixer 48 and the paper is carried out to a paper tray 49.In this process, a paper is supplied from a paper cassette 50 and ischucked at the top of the paper by a chucking mechanism 52 on thetransfer drum 51 in order to prevent a shift of position on the imagetransfer.

FIGS. 2 and 3 show a whole block diagram of the control system of thedigital color copying machine. The image reader 100 is controlled by animage reader controller 101. The controller 101 controls the exposurelamp 12 via a drive I/O 103 according to a position signal from aposition detection switch 102 which indicates the position of a documenton the platen 15 and controls a scan motor driver 105 via a drive I/O103. The scan motor 11 is driven by the scan motor driver 105.

On the other hand, the image reader controller 101 is connected via abus to an image controller 106. The image controller 106 is connected tothe CCD color image sensor 14 and the image signal processor 20. Theimage signal from the CCD color image sensor 14 is processed by theimage signal processor 20.

The main body 200 includes a printer controller 201 for controlling thecopying action and a print head controller 202 for controlling the printhead 31. The printer controller receives analog signals from varioussensors 44, 60 and 203-205 for automatic density control. Various datainputted with an operational panel 206 are sent to the printercontroller 201 via a parallel I/O 207. The printer controller 201 isconnected to a control ROM 208 storing a control program and a data ROM217 storing various data. The printer controller 201 controls a copyingcontroller 210 and the display panel 211 according to the data from theoperational panel 206 and the data ROM 209 under the contents of thecontrol ROM 208. Further, the printer controller 201 controls highvoltage units 214 and 215 for the grid voltage of the sensitizingcharger 43 and for the developer bias voltage of the developer 45a-45d.

The print head controller 202 acts according to the control programstored in the control ROM 216. The print head controller 202 isconnected to the image signal processor 202 of the image reader 100 viaan image bus and performs gamma correction on the basis of the imagesignal received via the image data bus with reference to a conversiontable stored in the data ROM 217. Further, a dither processing isperformed if necessary to express gradation. Then, the print headcontroller 202 controls the laser diode controller 220 via the drive I/O218 and a parallel I/O 216, and the laser diode controller 220 controlsthe emitting of the laser diode 221. Further, the print head controller202 is synchronized with the printer controller 201 and with the imagesignal processor 20 to each other via the buses.

FIG. 4 shows a perspective view of a reading device, in which a surfaceof a document placed on the platen 15 is illuminated by the light source(halogen lamp) 12 having an optical spectrum of three wavelengths (R, Gand B). The light reflected from the document is focused with the rodlens array 13 linearly on the light-receiving plane of the CCD sensor14. The optical system including the rod lens array 13, the light source12 and the CCD color image sensor 14 is moved in the direction of thearrow shown in FIG. 1, and the optical information of the document isconverted to an electrical signal by the CCD color image sensor 14.

Further, a white standard plate 16 and a black standard plate 17 for theshading correction are provided adjacent a side of the platen 15.

(b) structure of image signal processor

FIG. 5 shows a block diagram of the image signal processor 20, and theprocessing of the image data from the CCD color image sensor 14 via theimage signal processor 20 to the print head controller 202 is explainedbelow.

In the image signal processor 20, the image signal obtained by thephotoelectric conversion by the CCD sensor 14 is converted tomulti-value digital image data of r₇₋₀, g₇₋₀ and b₇₋₀ of red, green andblue of the three primary colors with an A/D converter 61. The convertedimage data is normalized with shading correction by a shading correctionsection 62 to generate image data of R₇₋₀, G₇₋₀ and B₇₋₀ of red, greenand blue. Then, the image data are converted to data V₇₋₀, W₇₋₀ and H₇₋₀of value, chroma and hue by an HVC converter 64, and the converted datais sent to an HVC line memory interface 66.

On the other hand, the image data of R₇₋₀, G₇₋₀ and B₇₋₀ which areoutputted as image data of RS₇₋₀, GS₇₋₀ and BS₇₋₀ by the HVC converter64 are reflection data of the document. Then, the image data RS₇₋₀,GS₇₋₀ and BS₇₋₀ are converted by a density conversion section 68 todensity data DR₇₋₀, DG₇₋₀ and DB₇₋₀ of the actual image according tologarithmic conversion. The converted data are sent to a colorcorrection section 72 and to an RGB line memory interface 70.

Further, a regional discrimination section 74 decides the region and thecolor according to the data V₇₋₀, W₇₋₀ and H₇₋₀ of value, chroma andhue, and sends UCR/BP ratios to the color correction section 72 andsends MTF data and a control data EDG to an MTF correction section 78.Further, the data V, W, H are decided as to the color in a colordecision section 76, which sends a signal CCS to the color correctionsection 72.

The color correction section 72 processes the black data generation andthe masking at the same time. That is, a black data is generated and theblack data is subtracted from the density data DR, DG and DB, while thedensity data DR, DG and DB are converted to data of the threereproduction colors, cyan, magenta and yellow.

Further, the MTF correction section 78 selects a digital filteraccording to a signal from the region discrimination section 74 toconduct optimum smoothing processing or edge emphasis processing. Colorblur correction and contour extraction are also performed.

Next, a magnification and remove section 80 changes the magnification.Further, a gamma correction section adjusts the color balance and sendsthe obtained data to the print head controller 202.

FIG. 6 shows a timing generator 84 which generates timing signals forthe image signal processor 20. The timing generator 84 generates a drivesignal for CCD in the horizontal (main scan) direction, a drive signalfor CCD in the vertical (subscan) direction, a timing signal of thedigitalization of the A/D converter 61 and image processing standardsignals.

FIG. 7 shows a circuit diagram of the HVC line memory interface 66. Theimage data V₇₋₀, W₇₋₀ and H₇₋₀ and an average value V_(E) sent from theHVC converter 64 are stored in the H,V,C line memory 104 via a selector100 and a bi-directional buffer 102, and are read via the bi-directionalbuffer 102 and a bus gate 106 by a CPU 140 (FIG. 9) which controls theimage signal processor 20. As shown in Table 1, the kinds of write dataand the address are controlled via an address counter 108 and a selector110.

                  TABLE 1                                                         ______________________________________                                         ##STR1##    3,2LDMPX   line memorywrite data into H, V,                      ______________________________________                                                               C,                                                     "L"         0          H.sub.7-0 (hue)                                                    1          V.sub.7-0 (value)                                                  2          W.sub.7-0 (chroma)                                                 3          V.sub.E7-0 (change in value)                           "H"         --         none                                                   ______________________________________                                    

FIG. 8 shows a circuit diagram of the RBG line memory interface 70.Image data DR₇₋₀, DG₇₋₀ and DB₇₋₀ sent from the density conversionsection 68 are stored in the RGB line memory 124 via a selector 120 anda bi-directional buffer 122, and are read via the bi-directional buffer122 and a bus gate 126 by the CPU 140 (FIG. 9). As shown in Table 2, thekinds of write data and the address are controlled via an addresscounter 128 and a selector 130.

                  TABLE 2                                                         ______________________________________                                         ##STR2##  1,0LDMPX   line memorywrite data into R, G, B                      ______________________________________                                        "L"       0          DR.sub.7-0 (red)                                                   1          DG.sub.7-0 (green)                                                 2          DB.sub.7-0 (blue)                                                  3          PD.sub.57-50 (output data to printer)                    "H"       --         none                                                     ______________________________________                                    

FIG. 9 shows a circuit diagram of a CPU peripheral circuit. The CPU 140monitors image data via the HVC and RGB line memory interfaces 66, 70,and conducts the detection of document size, the detection of systemanomaly and the automatic control of the signals of the CCD sensor 14.Further, the CPU 140 sets parameter signals according to read mode oredit mode of image processing. That is, various kinds of parametersignals are outputted via a decoder 142 through three parallel I/Ocircuits 144 to each component in the image signal processor 20.

(c) shading correction

FIG. 10 shows the shading correction section 62 which will be explainedlater. The shading correction section 62 corrects the black and whitelevels on the image data read by the CCD sensor 14 and performs thenormalization according to the following formula:

    DOUT=(DIN--BK).255/WH,

wherein DIN denotes the document read data r, g, b of the three primarycolors, WH denotes the read data of the shading standard white plate 16and BK denotes the black level data of the CCD sensor 14. The correctionis performed independently for each of the color data r, g, b.

The black level correction corrects the scattering of backgroundsensitivity level of each dot of the CCD sensor 14. Therefore, each ofthe read data r, g and b of the CCD sensor 14 are stored in a FIFOmemory 162 which is a line memory of one line, when there is no incidentlight. Then, the data WH are subtracted from the document read data DIN.This processing also removes the offsets of the digital signals.

The standard read data for the black level correction may be obtained bereading the white standard plate 16 (FIG. 4) after turning off theexposure lamp 12. Moreover, the standard read data may also be obtainedby reading the black standard plate 17 (FIG. 4).

On the other hand, the white level correction removes the nonuniformityof the sensitivity of the elements of the CCD sensor 14 and thescattering in the main scan direction of light distribution of theoptical system. Then, the read data WH of the white standard plate 16 ofshading correction are stored in the FIFO memory 182, and the documentread data DIN are multiplied with the inverse of the read data WH forcorrection. This processing has a function to normalize the output ofthe CCD sensor 14 of the three primary color data. Therefore, it alsoperforms the white balance correction in order to make the ratio of R,G, B data of a white document constant.

One of the problems of the shading correction is that when the standarddata for shading correction is determined according to one line ofsampling data and the sampling data include image noises, image noisesalways appears at the same pixel positions on the image data after theshading correction.

There are many factors which generate image noises such as power supplynoises in the system or cross-talk noises in the clock system. If thedynamic range (reference voltage) of the A/D conversion is about 2.39 Vand it is converted to an 8-bit data, 1 LSB corresponds to about 9.3 mV.The image noises worsen the S/N ratio for image data before the A/Dconversion.

Then, if the standard data BK and WH for correction are determined withuse of sampling data of a plurality of lines, image noises may bereduced.

One method of this type of the correction is to provide a memory for aplurality of lines to determine the standard data for correction. On thecontrary, in this embodiment, the correction is performed by using aline memory of one line and by obtaining a cumulative average of thedata of a plurality of lines.

In this embodiment, as shown in Tables 3 and 4, four mode are providedas to the corrections of the BK and WH levels: initial value mode, datageneration mode, hold mode and correction mode. These modes are selectedaccording to a mode selection signal SH₀₋₃ supplied by the CPU 140 (FIG.9) for each of the corrections of the BK and WH levels.

                  TABLE 3                                                         ______________________________________                                                 BK level       selecter selecter                                     SH.sub.1,0                                                                             correction mode                                                                              160      170                                          ______________________________________                                        0        initial value  C        B                                                     mode                                                                 1        data generation                                                                              B        B                                                     mode                                                                 2        hold mode      A        B                                            3        correction mode                                                                              A        A                                            ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                 WH level       selecter selecter                                     SH.sub.3,2                                                                             correction mode                                                                              180      190                                          ______________________________________                                        0        initial value  C        B                                                     mode                                                                 1        data generation                                                                              B        B                                            2        hold mode      A        B                                            3        correction mode                                                                              A        A                                            ______________________________________                                    

In the initial value mode, initial values are stored in the FIFOmemories 162, 182 of line memories for one line.

In the data generation mode, the data in the FIFO memory and thestandard data received successively are averaged with weights, and thestandard data for correction are generated at a high precision with acumulative average technique.

In the correction mode, the standard data for correction obtained in thedata generation mode are outputted and held at the same time to correctthe BK and WH levels for the document image signals.

Further, in the hold mode, the generated standard data for correction isheld while no correction is performed. This mode is used to accessalways the line memory which has a DRAM structure.

Next, the circuit of the shading correction section 62 shown in FIG. 10will be explained. The shading correction is performed for a pluralityof lines with use of the line memory of one line.

The image signal DIN₇₋₀ is sent via the C terminal (for the initialvalue mode) of a first selector 160 to the FIFO memory 162 for the BKlevel of DRAM structure, and the output signal of the FIFO memory 162 issent to the A terminal (for the hold and correction modes) of the firstselector 160. Further, the image signal DIN₇₋₀ is multiplied with 1/4 ina multiplier 164 and the product is sent to an adder 166, while theoutput of the FIFO memory 162 is multiplied with 3/4 in anothermultiplier 168 and the product is also sent to the adder 166. The adder166 obtains the sum (weighted average) of the two values and sends it tothe B terminal (for the data generation mode) of the first selector 160.The image signal DIN₇₋₀ is also received by a subtracter 172 while asecond selector 170 selects either of the output of the FIFO memory 162or "00" (correction mode) so as to be sent to a subtracter 172. Thesubtracter 172 outputs the image signal (DIN--BK) corrected by the blacklevel BK to a third selector 180, to a multiplier 184 and to amultiplier 192 only in the correction mode.

The image signal (DIN--BK) corrected by the black level BK is sent viathe C terminal (for the initial value mode) of the third selector 180 tothe FIFO memory 182 of DRAM structure for the WH level, and the outputsignal of the FIFO memory 182 is sent to the A terminal (for the holdand correction modes) of the third selector 180. Further, the imagesignal is multiplied with 1/4 in the multiplier 184 to be sent to anadder 186, while the output signal of the FIFO memory 182 is multipliedwith 3/4 in another multiplier 188 to be sent to the adder 186. Theadder 186 obtains the sum (weighted average) of the two values and sendsit to the B terminal (for the data generation mode) of the thirdselector 180. The image signal corrected by the BK level is received bya subtracter 192, as mentioned above, while a fourth selector 190selects either of the output of the FIFO memory 182 or "FF" (except thecorrection mode) to be sent to an inverse table 194. The inverse table194 outputs the inverse thereof (l/WH) to a multiplier 192. Themultiplier 192 multiplies the inverse with the image signal (DIN--BK) tooutput a gradation signal DOUT corrected with the black level BK andwith the white level WH.

The selection of each selector 160, 170, 180, 190 is performed accordingto the mode selection signal SH₁,0, SH₃,2 (refer Tables 3 and 4). Forthe black level correction, in the initial value mode (SH₁,0 =0), thefirst selector 160 selects the input at the C level, while the thirdselector 170 selects the input at the B terminal ("00"). Therefore, theimage signal DIN₇₋₀ as received are stored in the FIFO memory 162.

In the data generation mode (SH₁,0 =1), the first selector 160 selectsthe input at the C terminal, while the third selector 170 selects theinput at the B terminal ("00"). Therefore, the weight-average of theimage data of a plurality of lines are stored in the FIFO memory 162 asthe standard data for correction. In this embodiment, the data of theFIFO memory 162 to be inputted as initial values are averaged at a ratioof 3:1 with the standard data (DIN₇₋₀ =BK₁₇₋₁₀) received successively.Therefore, the black level converges at the sixteenth linetheoretically.

In the correction mode (SH₁,0 =3), the first selector 160 selects theinput at the A terminal, while the third selector 170 selects the inputat the A terminal (a value of the black level). That is, the FIFO memory162 holds the standard data for correction, while the subtracter 172corrects the black level for document image signals.

In the hold mode (SH₁,0 =2), the first selector 160 selects the input atthe A terminal while the third selector 170 selects the input at the Bterminal ("00"). Therefore, the FIFO memory 162 holds the standard datafor correction, while the subtracter 172 does not perform thecorrection.

The correction of the white level is also performed similarly.

Because this circuit has two FIFO memories as line memories, the writeand the read can be performed independently of each other.

Further, the circuits for the multipliers 164,168, 184 and 188 becomesimpler if the weight averages of the data of a plurality of lines areset to be 1/2n (wherein n=1, 2, . . . ) for the standard data for thecorrection to be received successively.

FIG. 11 shows the black correction block of the shading correctionsection 62 in detail as well as flip-flops 174 and the FIFO memory 162.The flip-flops 174 are used to perform synchronization with use of clockto control the pixel position (for example by delaying a signal WE byfour dots) when the weight-average and normalization correction areperformed as to the line memory 162.

FIGS. 12 and 13 shows a timing chart at the points (a)-(i) in FIG. 11,wherein a1, a2, a3, . . . denote standard data received successivelywhile f1, f2, f3, . . . denote the output values of the first selector160. Further, (c) denotes the weighted average and (i) denotes thecorrection value outputted by the subtracter 170.

In a modified example, an average of the standard data for thecorrection of one line may be stored beforehand in a line memory, andthe CPU may read it to set as the initial values. In the flow to beexplained later, the correction value (or the average) obtained in thelast time is used as the initial values in the shading correction at thesecond time or later. Though the use of such initial values takes alittle longer time for the conversion, similar results can be realized.

In another modified example, the magnification factor of the multiplier168 can be set to be "1" and a simple average of standard data of aplurality of line for correction. Foe example, the magnification of themultiplier 164 is set to be 1/16 and standard data of sixteen lines forcorrection are added in a FIFO memory. Then, a standard data can begenerated as an average of the standard data of the sixteen lines.

The shading correction circuit may be provided for each color resolutiondata of a document to be inputted. Then, red, green and blue can becorrected independently.

Further, the standard data may be corrected only at the white side or atthe black side. For example, only the white side may be corrected for anachromatic image.

In the above-mentioned shading correction, both BK and WH levels arecorrected independently, and each correction data are generated by usingcumulative average of the data of a plurality of lines. Therefore, thecorrection can be performed at a high precision. However, the timeneeded to generate the correction data becomes longer due to theaddition of the correction of the BK level and the addition of thecumulative average processing. Especially, in case of multi-scan (forexample, four scans of cyan, magenta, yellow and black for a full colorimage or copies of two or more sheets), it need a long time forcorrection. The copy time is also affected by the rise characteristicsof the exposure lamp. Then, these factors hinder the smooth copyoperation.

Then, in a modified example to be explained next, the time for thecorrection is shortened by performing only the cumulative averageprocessing of the correction data at the WH side in case of multi-scan.As to the BK level, the hold values obtained in the initialization whenthe power supply is turned on are used or the BK level is not updatedbecause the BK level is not affected by the environmental conditions,but is mainly ascribed to the scattering of the characteristics of theelements in the CCD sensor 14. On the other hand, the WH level needscorrection because the sensitivity of the CCD sensor 14 changesgradually due to the increase in temperature at the surroundings causedby the turning on and off of the exposure lamp in a multi-scan and thiscauses for example a fog. The correction in the example, the correctiondata at the WH level is updated for each scan to suppress the effect ofthe sensitivity change, while the corrected data obtained by thecumulative average is used always. Therefore, the shading correction canbe performed at a high precision.

FIGS. 14 and 15 show a flow of the shading correction, and FIG. 16 showsa sequence for the shading correction.

In this flow, when the power supply is turned on, the hold mode is setas to the corrections of the BK and WH levels (step S2). Next, the CCDsensor 14 is driven (step S4), and the initial value mode is set as tothe BK level correction, and the initial values are set in the FIFOmemory 162 (step S6). Next, the data generation mode is set as the to BKlevel correction (step S8), and the standard data inputted successivelyare averaged cumulatively and the averages are stored again in the FIFOmemory 162. When the averages are generated, the correction mode is setas to the BK level correction (step S10), and the standard data for thecorrection are sent for the correction of the WH level. Next, theexposure lamp 12 is turned on to illuminate the standard white plate 16(step S12). Next, the initial value mode is set as to the WH levelcorrection and the standard white plate 16 is read. Then, the readvalues are inputted as the initial values in the FIFO memory 182 (stepS14). Further, the data generation mode is set as to the WH levelcorrection (step S16), and the standard data inputted successively areaveraged cumulatively with weights and the averages are stored again inthe FIFO memory 182. When the averages are generated, the hold mode isset as to the BK and WH level corrections (step S18), and the correcteddata are held. Then, the exposure lamp 12 is turned off when the readcompletes, and the CCD sensor 14 is stopped to be driven (step S20).

Then, a copy request is waited (step S22). If a copy is performed, theinitial values obtained previously are used as to the BK level.

If a copy request is decided to be received (YES at step S22), the CCDsensor 14 is driven (step S24), the correction mode is set as to the BKlevel correction (step S26), and the data are outputted from the FIFOmemory 162 as the standard data for the BK level correction. Then, theexposure lamp 12 is turned on to read the standard white plate 16 to bestored in the FIFO memory 162 (step S28). Next, the data generation modeis set as to the WH level correction (step S30), the standard datainputted successively are averaged cumulatively with weights and theaverages are stored again in the FIFO memory 182. Then, the correctionmode is set as to the WH level correction (step S32), and the data ofthe FIFO memory 182 are outputted as the standard data for thecorrection. Then, a document is scanned to read an image (step S34). Atthis time, the shading correction with use of the corrected BK and WHlevels is performed. Then, the exposure lamp 12 is turned off, and theread completes (step S36).

Next, it is decided if multi-scan is performed or not (step S38). If themulti-scan is decided to be performed, the flow returns to step S28 andthe WH level is corrected again, and then the document is read.

If multi-scan is decided not to be performed, the CCD sensor 14 isstopped to be driven (step S40), and the hold mode is set as to the BKand WH level corrections to hold the corrected BK and WH levels (stepS42). Then, the flow returns to step S22, and a next copy request iswaited.

As explained above, the standard data for the correction are generatedby using a different sequence in a multi-scan from other cases. On theother hand, in order to shorten the copy time, the standard correctiondata of the black level correction may be used for example only afterthe second scan or later in a multi-scan.

(d) HVC conversion section

The read data r, g and b of red, green and blue are converted to hue Hand chroma W in order to process the image data in the HVC conversionsection 64.

(d-1) R, G, B read position correction

FIG. 17 displays the CCD sensor 14 schematically. The CCD sensor 14includes a linear, sequential array of pixels of red (denoted as R),green (denoted as G) and blue (denoted as B), and the directions of thepixels are inclined by 45° to reduce moire patterns. Then, at an edgeportion of an image, the read position is different for each color of R,G and B. Therefore, the color difference signals (W_(R), W_(B)) cannotbe separated correctly. In order to reduce this phenomenon, the readposition is corrected on the color resolution data after the shadingcorrection by using G pixels as basis, as follows:

    R.sub.n =(5/8)r.sub.n +(3/8)r.sub.n-1,

    G.sub.n =(3/4)g.sub.n +(1/4)(g.sub.n-1 +g.sub.n+1),

and

    B.sub.n =(5/8)b.sub.n +(3/8)b.sub.n+1,

wherein n denotes the pixel position in the transfer direction. (Theread position correction is not needed for some kinds of CCD sensors. Inthe diagram of FIG. 5, the read position correction is omitted forsimplicity, and the output data of the shading correction section 62 arerepresented as R, G, B.)

The color resolution signals R, G, B obtained as explained above areconverted next to the data of H (hue), V (value) and W (chroma). Thisconversion is performed for precise image recognition processings to beconducted later such as color change, automatic control of the UCR/BPratios, automatic MTF control and edge decision of achromatic color. Therelative luminous factor has a large weight at green and it is generallyapproximated as a ratio of R:G:B=0.229:0.587:0.114 with C light sourceand 2° field. The color difference signals (WR, WB) are obtained fromthe color resolution data and the value signal (V) as

    W.sub.B =B-V,

and

    W.sub.R =R-V.

Therefore, V, W_(B) and W_(R) can be calculated as a matrix calculation.##EQU1##

FIGS. 18(b) shows the read data of an image of black and white displayedin FIG. 18(a). If the corrected data displayed in FIG. 18(c) is comparedwith the as-read data of R, G and B displayed in FIG. 18(b), it is foundthat the change positions of the three colors agree to each other afterthe position correction. Then, color blur is also reduced as explainedlater.

(d-2) separation of chroma and hue

The two color difference signals WR and WB designate signals of theorthogonal coordinates in the hue plane in the color space. When theseare transformed into the polar coordinates, as shown in the Munsellcolor diagram shown in FIG. 19, the hue can be separated well. Thelength of the vector of a data designates the chroma W (or the chromadecreases in the direction to the center), while the angle designatesthe hue H. Therefore, the chroma signal (W) and the hue signal (H) canbe calculated as follows:

    W=(W.sub.R.sup.2 +W.sub.B.sup.2).sup.1/2 (however, W=255 if W≧256)

and

    H=(256/360) tan.sup.-1 (W.sub.R /W.sub.B).

(d-3) circuit of HVC conversion section

FIG. 20 shows a block diagram of the HVC conversion section 64. Theposition correction section 300 which is a table for the above-mentionedcorrection of the read position converts the read data r, g, b to thedata RS, GS and BS of the three colors. The value/color differenceseparator 302 which is a table for the above-mentioned matrixcalculation converts the data R, G, B to V and W_(R), W_(B). The signalsW_(R) and W_(B) are converted to the chroma signal W in a chromaextraction circuit 304, while they are converted to the hue signal H ina hue extraction circuit 306.

FIG. 21 shows a circuit diagram of the chroma extraction circuit 304.After the signals W_(R) and W_(B) are converted to absolute values inthe absolute value circuits 320 and 322, they are squared by squarecircuits 324 and 326 and added in an adder 326, respectively. The sum isconverted with a square table 330 to a root or the chroma W isoutputted.

FIG. 22 shows a diagram of the hue extraction circuit 306, wherein thesignals W_(R) and W_(B) are converted to the hue signal H with a tan⁻¹table 346.

(e) density conversion

In the density conversion section 68, the output data of the CCD sensor14 is converted to data which has a linear characteristic for thedocument density (OD) observed by human naked eyes. The output of theCCD sensor 14 has a linear characteristic for the incident intensity(=reflection ratio (OR) of document). On the other hand, the reflectance(OR) of document and the document density (OD) have a relation: -logOR=OD. Then, the nonlinear read characteristic of the CCD sensor 14 isconverted to a linear characteristic with a reflectance/densityconversion table 360.

FIG. 23 shows the density conversion section 68. The image data RS, GS,BS and the value signal V are first converted with a density conversiontable 360 to density data RL, GL, BL and VL when a LOG signal isreceived.

After the density conversion, in a negative/positive reversal circuit362, a negative or positive output is selected to send the data DR, DGand DB. The selection is performed according to a control signal NEGAwhich denotes the output of the as-received density data (B output) atthe high level or the output of the inverted density data (A output) atthe low level. If the A output is selected, the circuit 362 reverses theinput data. Further, the output of the negative/positive reversalcircuit 362 and "00" are selected by a selector 364 according to aneffective document area signal NHD1. That is, outside the area forreading a document, the output is set to be white ("00") irrespective ofthe negative/positive output.

Further, the value data V is converted to a monochromatic color densitydata DV.

(f) region discrimination

As will be explained later, the color correction section 72 controls theblack reproducibility and the brightness of colors and also borderingprocessing. Further, the MTF correction section 78 performs edgeemphasis processing. These processing have to be controlled according tothe nature of an image. Then, by using the data obtained on the HVCconversion, the region discrimination section 74 performs the automaticUCR/BP ratios control according to the chroma data, the automatic edgeamount control according to the value change and the setting and theregion discrimination on special processing (color blur correction) atan achromatic color edge portion.

FIG. 24 shows a block diagram of a section 94 related to the regiondiscrimination in the image signal processor 20. The image data areconverted to the density data DR, DG, DB of red, green, blue in thedensity conversion section 68 and are converted to the value V andchroma W in the HVC conversion section 64. The value signal V is used todetect an edge in an edge detection section 400, 402, 420, and theresult is sent to the color blur correction table 620. Further, the edgeamount EGis converted in an MTF control table 412 to MTF data A which issent to a multiplier 622.

The MIN value of the density data detected by an MIN detector 502 issent to the UCR/BP processor 500(72). On the other hand, after thesignal W is smoothed by a smoothing filter 430, the UCR/BP ratiosdetermined according to the result are sent by the UCR/BP control table432 to the UCR/BP processor 500(72). By using the UCR/BP ratiosdetermined as explained above, the density data DR, DG and DB of red,green and blue are converted to the signals C, M, Y and K of cyan,magenta, yellow and black in the masking processor 510.

On the other hand, the MIN value is also sent to an achromatic colordecision section 442 to decide an achromatic color or not on thesmoothed chroma signal W, and the result is sent to the color blurcorrection table 620.

In the color blur correction table 620, the secondary derivative of thesignals C, M, Y and K obtained with a Laplacian filter 610 is correctedaccording to the decisions of the achromatic color and of the edge, andthe corrected value is multiplied with the output of the MTF controltable 412. Further, the obtained value is added by an adder 608 with thesmoothed signal C, M, Y and K, and the sum is outputted.

FIG. 25 shows a block diagram for concisely showing portions related tothe region discrimination for the automatic MTF correction in themonochromatic mode. An edge component of the read data of red, green andblue is detected with primary differential filters 400, 402 and absolutevalue detectors 404, 406, and the detected value is converted by the MTFcontrol table 412 to the MTF data A.

(f-1) circuit of region discrimination section

FIGS. 26 and 27 show circuit diagrams of the region discriminationsection 94. Edge components are detected from the value data V receivedfrom the density conversion section 68 by using a primary differentialfilter 400 in the main scan direction and a primary differential filter402 in the subscan direction, and the absolute value detectors 404 and406 send the absolute values of the edge components, respectively.Further, an average circuit 410 averages the absolute values to generatean average V_(E). The average V_(E) and the sharpness setting valuesSHARP₅₋₃ are converted by the MTF control table 412 to the MTF data A.

The two absolute values are also received by comparators 420 and 422,and they are compared with a threshold value REF. If either of theabsolute values is larger than the threshold value REF, an edge signalED is outputted via an OR gate 424 to an AND gate 426, which sends aWAKU signal if an EGEN1 signal is received.

Further, the chroma signal W is smoothed through a smoothing filter 430which outputs a signal WS, which is next converted to UCR/BP data by aUCR/BP control table 432.

Further, the minimum data MIN received from the color correction section72 is converted to the BK level (D) by a BK level reference table 440and the BK level is compared with the smoothed signal WS received fromthe smoothing filter 430 by a comparator (achromatic decision section)442, which sends a signal BK to an AND gate 444 if the BK level islarger or there is a black edge. The signal BK is further outputted fromthe AND gate 444 to an AND gate 448 if the edge signal EG is outputtedfrom the OR gate 424. When the WAKU signal (bordering edition area), theCH signal (color change edition area) or the MON signal (monochromaticcolor edition area) is not generated, the NAND gate 446 outputs the EDsignal. Further, when the color mode signal CMY/K is outputted, the ANDgate 448 sends an achromatic color edge decision signal EDG.

The BK level reference table 440 determines the threshold level forbinarizing the WS signal as shown in FIG. 28, when the minimum value ofDR, DG and DB is received. Because the focus depths of the incidentlight R, G and B are different from each other due to the chromaticaberration of lens, the color difference signals W_(R) and W_(B) of anachromatic color document having higher spatial frequencies becomeslarger than usual. Therefore, the bi-level threshold value is controlledaccording to the minimum MIN of the black level output.

The above-mentioned primary differential filters 400 and 402 havestructures shown in FIG. 29. That is, the data of four successive linesare stored successively in line memories 414a, 414b, 414c and 414d.Further, when the data of a fifth line is received, the calculations ofthe primary differential filters 400 and 402 are conducted on the dataof the (5×5) pixels. In the filter 400 for the main scan direction, thevalues displayed in the filter is multiplied with the input data only atthe two ends in the main scan direction, and the products are summed.Then, as to the pixel at the center of the 5×5 pixels, the value V_(H)of the primary derivative (edge amount) is obtained in the main scandirection. As to the other filter 402, a similar calculation isconducted to obtain an edge amount V_(V) in the subscan direction.

On the other hand, FIG. 30 shows a structure of the smoothing filter 430for smoothing the W signal received from the color correction section72. That is, the data of three successive lines are stored successivelyin line memories 434a, 434b and 434c. Then, the calculation of thesmoothing filter 430 is conducted on the data of the (3×3) pixels. Thevalues displayed in the filter is multiplied with the input data, andthe products are summed. Then, as to the pixel at the center of the 3×3pixels, the smoothed value WS is obtained.

(f-2) automatic control of under color remove/black painting

As will be explained later in detail, the color correction section 72detects K'=MIN(DR, DG, DB) as the black data K'. Then, the density dataDR, DG and DB of the three colors are subtracted by α·K' (under colorremove amount), while β·K' (black paint amount) is sent as the K amountused to generate the black data. "α" represents the UCR ratio used fordecreasing the color data, while "β" represents the BP ratio used fordetermining the black amount. The UCR/BP ratios affect the chroma of thechromatic colors and the pureness of achromatic color.

The UCR/BP ratios have a trade-off relation with the colorreproducibility. That is, the reproducibility of black is improved byincreasing each of UCR/BP ratios (-α/β) because black is reproduced byusing pure black K'. On the contrary, the brightness of chromatic colorsare deteriorated because the output ratio of K' increases. As the UCR/BPratios increase to 100%, the change in the amplitude of the image dataDR, DG and DB after the under color is removed becomes extremely small.Then, the signal errors cannot be neglected and the image noises ofchromatic colors cannot be neglected also. Therefore, it is desirable tocontrol the UCR/BP ratios according to the chroma so that the UCR/BPratios decreased to 0% for a document of chromatic colors and increasesto 100% for a document of achromatic color. Then, an ideal colorreproduction processing can be performed or the pureness of achromaticcolor and the chroma of chromatic colors are not improved at the sametime.

However, as to a color image, such a processing does not necessarilyoperated well if the UCR/BP ratios are determined only from the blackamount K', because the image density changes according to a change inhue or in value. For example, when the color of an image changes fromwhite to red, an edge can be emphasized. On the other hand, when thecolor of an image changes from red to cyan, the hue changes anomalouslyat the edge. Therefore, it is better not to emphasize the edge. Forexample, an image such as the skin in a human face is affectedespecially largely. Therefore, only the change in the value of an imagehas to be subtracted well for the automatic control. Then, in thepresent embodiment, the UCR/BP processing is conducted by using thechroma data.

The chroma signal W is sent to a 3×3 smoothing filter for smoothing inorder to suppress an extreme data change at an edge portion. Next, thesmoothed chroma signal WS is converted to UCR/BP data DB₇₋₀ with theUCR/BP control table 432, as shown in FIG. 31, and the data is sent tothe color correction section 72. In other words, the UCR/BP data D ischanged linearly between 0% to 100% against the amplitude of the chromasignal WS, except near 0%.

In the color correction section 72, the automatic UCR/BP processing aswell as the masking processing are performed, as will be explainedlater.

(f-3) automatic control of edge emphasis

The human sense on the density of image has the -log characteristicagainst the incident light amplitude. However, in an edge portion of animage, the sense responds to a change not in density, but in value.Therefore, when the edge emphasis is processing on the data after thedensity conversion, following problems arise: As to an edge portion of acharacter or a narrow line, the data changes more smoothly than beforethe density conversion, and sufficient edge emphasis is not performed.Further, the data change at the high density side is large so that anedge is easily detected at the high density side and even image noisesare emphasized. This is ascribable to that the slope of the -logcharacteristic is small at the low density side and large at the highdensity side. As to a full color image, the hue changes if the edgeemphasis is performed on the data after the color correction.

Therefore, in order to reduce the above-mentioned problems, the changein the value component of an image is detected, and the edge emphasisamount is controlled according to the detected value.

Then, first, the read data R, G, B are converted to the value V and thevalue data V₇₋₀ is sent to the primary differential filters 400 and 402for the main scan direction and for the subscan direction, respectively,in order to extract the changes in each direction. Then, an averageV_(E7-0) of the absolute values of the changes is obtained. The resultis converted with the MTF control table 412, as shown in FIG. 32, to theMTF data A (D₇₋₀) which is sent to the MTF correction section 78. InFIG. 32, the signal SHARP₅₋₃ denotes the BANK number of the table, andthe emphasis amount (D₇₋₀) is adjusted according to the sharpnesssignal. FIG. 32 shows an example wherein the emphasis is performed witha factor 1.75 when the SHARP="7" and with a factor 0.25 when theSHARP="0". That is, the average V_(E) of the absolute values of edgeamounts is converted linearly to the MTF data A except near zero of thesignal V_(E).

FIG. 33 shows the value distribution, the output of the primarydifferential filter 502 in correspondence to the value distribution andthe absolute value thereof detected by the absolute value detector 404when an original image shown at the top is read at the central line. Theabsolute value is averaged and is converted to the MTF data A inresponse to the appropriate SHARP setting value. The result is next sentto the MTF correction section 78 wherein it is multiplied for edgeemphasis with the output of a Laplacian filter 610 of the density PD ofC, M and Y.

(f-4) decision of achromatic edge portion

In order to improve the reproducibility of a black character or a blacknarrow line in a full color image, it is better to prevent the colorblur at an achromatic edge portion. The color blur can be prevented byremoving the C, M, Y data and by performing the edge emphasis only forthe K data in an achromatic edge portion.

First, the absolute values, |VH₇₋₀ | and |V_(V7-0) |, are converted tobi-level data with the threshold value REF, and the edge emphasis isdecided with the comparators 420 and 422 in either of the main scan andsubscan directions (FIG. 26). If an edge is detected, ED is set to be"L".

Second, the signal WS is compared by the comparator 442 with the outputof the BK level reference table 440 by the comparator 442 or isbinarized in order to decide if the image has an achromatic color ornot. If it is decided to have an achromatic color, BKis set to be "L".

Third, only when ED="L" and BK="L", an achromatic edge decision signalEDG="L" is outputted via the AND gate 444 in order to conduct a specialprocessing (color blur correction) in the MTF correction section 78.

In the case of either of the monochromatic color edition area(MONO="L"), the color change edition area (CH="L") and the borderingedition area (WAKU="L"), the achromatic edge decision signal EDG is madeinvalid through the NAND gate 446 and the AND gate 448. This isperformed in order to prevent the effect of the color blur correction insuch edition areas.

As will be explained later, in the color blur correction in the MTFcorrection section 78, if the color resolution data for reproduction inthe MTF correction section 78 is black K, the achromatic edge decisionsignal EDG is made valid only when the color signal CMY/K signal is "L".This is because the usual area emphasis processing is performed. (ED="L"is a signal which shows an edge portion of an image, and WAKU isoutputted as the bordering edition area signal, wherein the signal EGEN1is a signal for the permission of the bordering edition.)

(g) color correction

Each color data C', M', Y' and K' of cyan, magenta, yellow and blacknecessary for reproducing a full color image is generated for each scanin the plane-at-a-time scheme, and a full color image is reproduced withfour successive scans. The printing of black is performed because theoverlay of cyan, magenta and yellow is difficult to reproduce blackbecause the reproduction of pure black is hard due to the spectralcharacteristic of each toner. Then, in the full-color printer, thereproducibility is improved in a full color image by using thesubtractive mixing of the data C', M' and Y' and the black paint of thedata K'.

(g-1) automatic UCR/BP processing

The black amount K is obtained in the color correction section 72 fromthe red, green and blue components DR, DG and DB which express thebrightness on a document. Because the data DR, DG and DB received fromthe density conversion section 68 are the density data of the R, G and Bcomponents, the they agree with the C', M' and Y' components of cyan,magenta and yellow which are complementary colors of red, green andblue. Therefore, as shown in FIG. 34, the minimum of DR, DG and DB arethe component wherein the C', M' and Y' on the document are overlayed,and the minimum of DR, DG and DB can be taken as the black data K'.Then, in the color correction section 72, the black data K'=MIN(DR, DG,DB) is detected.

When the reproduction color data C', M'and Y' are generated, the data K'is used to subtract α·K' (under color remove amount) from the data C',M' and Y' and to generate the black data K as β·K' (black paint amount).As explained before, α denotes the UCR (under color remove) ratio, whileβ denotes the BP (black paint) ratio, and these ratios are set in theregion discrimination section 74 not directly from the density data DR,DG and DB of red, green and blue, but from the chroma data WS obtainedby the HVC conversion of these data (refer FIGS. 26 and 31).

Further, in order to improve the brightness at the low density side inthe UCR/BP processing, certain levels d₁ and d₂ (K cut-off data) aresubtracted from the MIN (DR, DG, DB) before the multiplication with αand β, as will be explained later.

(g-2) masking processing

Further, the color correction section 72 corrects the transmissioncharacteristics of each filter R, G, B provided in the CCD sensor 14 andthe reflection characteristics of each toner of cyan, magenta and yellowfor the development in order to make matching with the ideal colorreproduction characteristics. As shown for example in the transmissioncharacteristics of a green filter in FIG. 35 and in the reflectioncharacteristics of a magenta toner in FIG. 36, each characteristic has anonideal wavelength region as displayed with oblique lines. Then, inorder to correct this characteristics as well as to perform theabove-mentioned UCR/BP processing, the linear masking correction isconducted.

That is, the MTF correction section 78 performs the masking calculationin order to reproduce an image from the full color input data. Themasking coefficients (A_(c),m,y, B_(c),m,y and C_(c),m,y) are set sothat the average color difference becomes minimum over the almost allcolor reproduction region. As the printing is repeated for each of thefour reproduction colors successively, the masking equation is carriedout by one line at one time. ##EQU2##

    K=β.{MIN(DR, DG, DB)-d.sub.2 }

(g-3) examples of automatic UCR/BP processing

FIG. 37 shows an example of R, G and B data along one line in a greyscale, while FIG. 38 displays a part of the results of the UCR/BPprocessing on the data shown in FIG. 37. In FIG. 37, the obtained dataR, G and B of the three colors are of about the same order. The densityof the grey scale changes from left (black) to right (white) gradually.However, since the chroma WS of the grey scale is zero, the UCR/BPratios are 100%. After the masking calculation, as shown in FIG. 38, theoutputs of C, M and Y become zero, and the density change of the greyscale can be expressed almost only with K. Thus, the colorreproducibility of the grey scale is improved.

Further, FIG. 39 shows an example of the R, G and B data along one linein an image of black and white shown in the upper portion of FIG. 39.FIG. 40 shows the density conversion data of the read data, and FIGS. 41and 42 show the results of the HVC conversion. As shown in FIG. 40, theC, M and Y values are large in the white portion of the image, while asshown in FIG. 42, the chroma of the image is small. Therefore, theUCR/BP ratios becomes almost 100%, and as shown in the results of theUCR/BP processing in FIG. 43, the C, M and Y outputs become small andthe image can be expressed almost only with K. Therefore, the blackreproducibility of an achromatic image can be improved.

Still further, FIG. 44 shows an example of the R, G and B data along oneline in an image of red and white shown in the upper portion of FIG. 44.FIG. 45 shows the density conversion data of the read data, and FIGS. 46and 47 show the results of the HVC conversion. As shown in FIG. 45, inthe white portion of the image, the C, M and Y values are large, whilein the red portion of the image, the C and K' values are large and the Mand Y data are also not small. As shown in FIG. 47, the chroma in thered portion is large. In the UCR/BP processing in this example, theUCR/BP ratios are set to be 0% if WS≧85 and 100(1-W/85) % if WS<85.Therefore, as shown in the results of the UCR/BP processing in FIG. 48,the K output vanishes and the image can be expressed only with chromaticcolors almost of cyan. Therefore, the brightness of a chromatic imagecan be improved.

(g-4) monochromatic color mode and color change mode

The color reproduced in the monochromatic color mode can be selectedamong red (R), green (G), blue (B), cyan (C), magenta (M), yellow (Y)and black (K). As shown in Table 5, the reproduction color is determinedby the state (C, M, Y, K) of the color resolution data to be sent to theprinter and the logic signal of WH, to generate the final data (MONO₇₋₀)in the monochromatic mode. That is, if WH="L", "00" is selected, whileif WH="H", DV₇₋₀ is selected.

                  TABLE 5                                                         ______________________________________                                                   state of color resolution data                                                C         M       Y       K                                        ______________________________________                                         reproduction                                                                             C                                                                                   ##STR3##    "L"   "L"   "L"                                 data       M     "L"         "H"   "L"   "L"                                  of         Y     "L"         "L"   "H"   "L"                                  monochromatic                                                                            K     "L"         "L"   "L"   "H"                                  color      R     "L"         "H"   "H"   "L"                                  data       G     "H"         "L"   "H"   "L"                                             B     "H"         "H"   "L"   "L"                                  ______________________________________                                    

The data for color change is also selected at the same time, and thefull color data (PD₇₋₀) can also be selected.

The full color data (PD₇₋₀), the monochromatic color data (MONO₇₋₀) andthe color change data are controlled according to the signals CHAN, CCSand MONO, as shown in Table 6.

                  TABLE 6                                                         ______________________________________                                         ##STR4##                                                                                ##STR5##                                                                              ##STR6##  selected data                                    ______________________________________                                        "L"       "L"     --        change data                                                 "H"     "L"       data of monochromatic                                                         data (MONO.sub.7-0)                                                 "H"       data of full color                                                            (PD.sub.7-0)                                      "H"       --      "L"       data of monochromatic                                                         data (MONO.sub.7-0)                                                 "H"       data of full color                                                            (PD.sub.7-0)                                      ______________________________________                                    

The selections of Tables 5 and 6 are performed in the color decisionsection 76 with use of a table (not shown).

(g-5) bordering processing

In the bordering mode, as shown in FIG. 49, the bordering of an image isperformed. As to an example of the read data of a document image (alarge character "A") shown at the top portion in FIG. 49, the primaryderivative (edge amount) of the value signal V is obtained, and theabsolute value thereof is detected. The edge signal ED is outputted ifthe absolute value is larger than a threshold value REF, and a borderedimage is outputted according the edge signal ED. That is, a finite widthof image is outputted around the edge portion (contour) of an image, andthe size of the original image in the bordered image becomes smaller alittle.

If EDGE1="L", the edge signal ED is permitted, and the bordering modedesignation signal (WAKU) is generated.

The color for bordering can be chosen among the seven colors, C, M, Y,R, G, B and K. The print color is determined according to the state (C,M, Y, K) of the color resolution data, as shown in Table 7, and theselector 532 sends the data according to the WCLR signal, the borderingedition permission signal EGN1, the edge signal ED and the borderingmode designation signal WAKU, as shown in Table 8.

Further, the width of the edge portion for changing the color may bechanged by an operator by providing an input means for specifying thethreshold value REF.

                  TABLE 7                                                         ______________________________________                                                   state of color resolution data                                                C      M        Y        K                                         ______________________________________                                        designated                                                                              C      "FF"     00     00     00                                    data      M      00       "FF"   00     00                                              Y      00       00     "FF"   00                                              K      00       00     00     "FF"                                            R      00       "FF"   "FF"   00                                              G      "FF"     00     "FF"   00                                              B      "FF"     "FF"   00     00                                    ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                         ##STR7##                                                                                ##STR8##                                                                               ##STR9##  WCLR   outputselector                           ______________________________________                                        "H"       --       "H"       --     selector                                                                      output                                    "L"       "H"      "H"       --     selector                                                                      output                                              "L"      "L"       "L"    "00"                                                                   "H"    "FF"                                      ______________________________________                                    

(g-6) circuit of color correction section

FIGS. 50 and 51 display the circuit diagram of the color correctionsection 72. First, in the under color remove/black paint section 500,the minimum MIN(DR, DG and DB) is detected from the density signals DR,DG and DB received from the density conversion section 68 by the minimumdetector 502. After the minimum is subtracted with a prescribed blackcut-off data "d" by a subtracter 504, the difference is multiplied withthe UCR/BP data (α/β) in a multiplier 506 to generate the black data K.Further, in a subtracter 508, the density data DR, DG, DB are subtractedwith the black data K to remove the under color.

In the masking calculation section 510, the color data DR, DG and DB ofthe three colors received from the subtracter 508 are multiplied withthe masking data (A_(c),m,y, B_(c),m,y, C_(c),m,y) by a multiplier 512,and the result is added by an adder 514 to generate the data C, M and Yof cyan, magenta and yellow. Further, the data and the black data K fromthe multiplier 506 are selected by a selector 516 according to the colorsignal CMY/K to send a data PD.

Further, in the color change and monochromatic color selection section520, the data PD is inputted to the A terminal of a selector 522. On theother hand, the value signal DV received from the density conversionsection 68 and "00" are selected according to the signal WH in aselector 524 to send a monochromatic signal MONO to the B terminal inthe selector 522. Further, the color change data is inputted to the Cterminal of the selector 522. These signals are selected according tothe signals MONO and CH by the selector 522. The signal CH is sent ifboth signals CHN and CCS are "L". In the bordering edition section 530,the input signals "00" and "FF" are selected according to the signalsWAKU and WCLR to generate a signal PD₁₇₋₁₀.

FIG. 52 shows a circuit diagram of the register section 540. In thisembodiment, two kinds of data can be set for the above-mentioned UCR/BPdata (α₇₋₀ /β₇₋₀), the masking data (A_(c),m,y, B_(c),m,y, C_(c),m,y)and the black cut-off data (d₇₋₀). Then, when a GCS0 signal isgenerated, a parameter MA₃₋₀ is decoded by a decoder 542, and when asignal WR is generated, the data sent by the CPU are stored in theregister 544, as denoted in FIG. 52. These data are sent to a selector546, and one of them is selected according to a selection signal MPX1 asmasking data (A_(c),m,y, B_(c),m,y, C_(c),m,y), UCR/BP data (α₇₋₀ /β₇₋₀)and black cut-off data (d₇₋₀).

As shown in Table 9, two kinds of these data, the masking data(A_(c),m,y, B_(c),m,y, C_(c),m,y), the UCR/BP data (α₇₋₀ /β₇₋₀) and theblack cut-off data (d₇₋₀), can be set in the register 544, in theaddress map of the CPU. If GCS0="L", the data are set at the rising edgeof the WR signal. If MPX1="L", the first kind of data are selected,otherwise the second kind of data are selected.

                  TABLE 9                                                         ______________________________________                                        HA.sub.3-0                                                                           contents to be set in register                                         ______________________________________                                        0      first masking coefficient                                                                        A.sub.c,m,y 9-7 = MD.sub.2-0                               for C(DR) term                                                         1      first masking coefficient                                                                        A.sub.c,m,y 6-0 = MD.sub.6-0                               for C(DR) term                                                         2      first masking coefficient                                                                        B.sub.c,m,y 9-7 = MD.sub.2-0                               for M(DG) term                                                         3      first masking coefficient                                                                        B.sub.c,m,y 6-0 = MD.sub.6-0                               for M(DG) term                                                         4      first masking coefficient                                                                        C.sub.c,m,y 9-7 = MD.sub.2-0                               for Y(DB) term                                                         5      first masking coefficient                                                                        C.sub.c,m,y 6-0 = MD.sub.6-0                               for Y(DB) term                                                         6      first UCR/BP ratios                                                                              α, β.sub.7-0 = MD.sub.7-0                7      first K cut data   d.sub.7-0 = MD.sub.7-0                              8      second masking coefficient                                                                       A.sub.c,m,y 9-7 = MD.sub.2-0                               for C(DR) term                                                         9      second masking coefficient                                                                       A.sub.c,m,y 6-0 = MD.sub.6-0                               for C(DR) term                                                         10     second masking coefficient                                                                       B.sub.c,m,y 9- 7 = MD.sub.2-0                              for M(DG) term                                                         11     second masking coefficient                                                                       B.sub.c,m,y 6-10 = MD.sub.6-0                              for M(DG) term                                                         12     second masking coefficient                                                                       C.sub.c,m,y 9-7 = MD.sub.2-0                               for Y(DB) term                                                         13     second masking coefficient                                                                       C.sub.c,m,y 6-0 = MD.sub.6-0                               for Y(DB) term                                                         14     second UCR/BP ratios                                                                             α, β.sub.7-0 = MD.sub.7-0                15     second K cut data  d.sub.7-0 = MD.sub.7-0                              ______________________________________                                    

The output data of the UCR/BP data (α/β) of the register 544 and theUCR₇₋₀ are selected further in a selector 548 according to a selectionsignal MPX0 to send UCR/BP data. The data UCR₇₋₀ is the chroma datareceived from the region discrimination section 74.

Further, Tables 10 and 11 display the bit definition of the masking dataand the UCR/BP data, respectively.

                  TABLE 10                                                        ______________________________________                                        bit  7       6      5    4    3    2    1      0                              ______________________________________                                        digit                                                                              1/2     1/4    1/8  1/16 1/32 1/64 1/128  1/256                          ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        bit  7      6      5     4    3     2    1     0                              ______________________________________                                        digit                                                                              1      1/2    1/4   1/8  1/16  1/32 1/64  1/128                          ______________________________________                                    

(h) MTF correction

The MTF correction section 78 performs smoothing processing, edgeemphasis processing and color blur correction.

(h-1) circuit of MTF correction section

FIG. 53 displays a circuit diagram of the MTF correction section 78. Theoutline of the image data processing can be understood with use of theblock diagram of FIG. 24.

The signal PD received from the color correction section 72 is smoothedby performing the weight average of the central pixel and pixels aroundit with use of two kinds of two-dimensional FIR type digital filters 600and 602. The output signals of the two filters and the signals notprocessed with the above-mentioned smoothing are received by a selector604, and are selected according to the signal SHARP₁,0. Thus, the imagenoises are reduced and the smoothing of image data is performed.

The signal PD received from the color correction section 72 is also sentto a Laplacian filter 610, which is called also as a secondarydifferential filter and acts to extract an edge component of the inputimage.

The output of the selector 612 is inputted in a Laplacian table (colorblur correction table) 620 as well as the signal EDG for color blurcorrection table and the selection signal MODE of read mode(photograph/standard mode). Then, as shown in FIG. 57, the output Dchanges according to the input value. The color blur correction isperformed if EDG="L" and MODE="H".

The output of the Laplacian filter 620 is next multiplied by amultiplier 622 with the MTF data A for edge control generated accordingto the value data V in the region discrimination section 74, and thenthe product is sent to an adder 608. As shown in FIG. 32, the MTF data Ais changed according to the edge amount detected by the primarydifferential filter.

Thus, an image data is outputted which has been subjected to thesmoothing and the edge emphasis according to the edge amount detected inthe edge image portion with the Laplacian filter. That is, in a flatportion of an image, the smoothing vanishes the unevenness of image,while in an edge image portion, the dullness of density change isprevented according to the edge detection amount with the Laplacianfilter. Further, the correction at an edge is adjusted according to themagnitude of the edge of the value detected with the primarydifferential filter in the HVC conversion section 64. Examples of thisautomatic MTF correction will be explained in (h-7).

An edge level decision circuit 630 compares an edge detection signal RAPreceived from the Laplacian filter 610 with the threshold value REF togenerate an image contour extraction signal RAP.

The image data and the EDG data are inputted to a selector 632, and theEDG data is selected if RAP="L" and EGEN2 (contour extractionsignal)="L".

FIG. 54 shows a register 640 which stores various control data and theperipheral thereof. The register 640 receives the data MD and the writesignal WR from the CPU, the MTF data B, REF and EDG are stored accordingto the decoded signal by a decoder 642 from the GCS1 and MA1,0 signals.The MTF data B stored in the register 640 and the MTF data A arereceived by a selector 644 and the selection is performed according to aMPX2 signal.

(h-2) smoothing

FIG. 55 shows the smoothing correction more concretely. As explainedabove, the signal PD received from the color correction section 72 issmoothed by performing the weight average of the central pixel andpixels around it with use of two kinds of two-dimensional FIR typedigital filters 600 and 602. The output signals of the two filters andthe signals not smoothed are received by a selector 604, and one of themis selected according to the signal SHARP₁,0. Thus, the image noises arereduced and the smoothing of image data is performed.

The data of four successive lines are stored successively in linememories 606a, 606b, 606c and 606d. Further, when the data of a fifthline is received, the calculation of the first smoothing filter 600 isconducted on the data of the (5×5) pixels, while the calculation of thesecond smoothing filter 602 is conducted on the data of the (3×3)pixels. In the calculations, the numerals displayed in the filters 600,602 are multiplied with the input data, and the results are summed.Then, as to the pixel at the center of the 5×5 pixels, the smoothedvalue is obtained. Table 12 shows the selection of the selector 604according to the selection signal SHARP₁,0.

                  TABLE 12                                                        ______________________________________                                        SHARP.sub.1,0                                                                              output of selector 604                                           ______________________________________                                        0            5 × 5 smoothing                                            1            3 × 3 smoothing                                            2,3          none                                                             ______________________________________                                    

(h-3) edge emphasis

The signal PD from the color correction section 72 is also inputted tothe Laplacian filter 610 which acts to extract an edge component of aninput image. As will be explained later, the sharpening (edge emphasis)of image is performed by adding the result of the processing of thefilter with the input image.

FIG. 56 shows the Laplacian filter 610. The filter is a 5×5 filter, andin the calculation, the numerals displayed in the filter 610 ismultiplied with the corresponding data and the products are summed.

The output of the Laplacian filter 610 is forced to be cleared byselecting "00" of the SHARP signal in a selector 612 (FIG. 53) if theedge emphasis is not needed.

(h-4) edge emphasis and sharpness mode

As shown in FIG. 57, the output of the selector 612 is inputted to theLaplacian table 620 as well as the EEG signal for color blur correctionand the selection signal MODE for read mode (photograph/standard), andthe filter output D is changed according to the input.

As shown in FIG. 57, in the photograph mode (MODE="L"), the output D isproportional to the input A. On the other hand, in the standard mode(MODE="H"), the output D is always -64 if EDG="L" (black edge emphasis)while the ordinary edge emphasis of the output D is performed if EDG="H"or the output D is constant at large and small values of the input A.

The output D of the Laplacian table 620 (FIG. 53) is further multipliedin the multiplier 622 with the MTF data for the edge control generatedfrom the value data V in the region discrimination section 74, and theproduct as well as the smoothing data of the original image from theselector 604 is inputted to the adder 608. Then, the smoothing data andthe edge amount are added and an image data processed for the edgeemphasis can be outputted. FIG. 58 shows the density conversion data ofan image shown at the top portion of FIG. 58 and the result of theprocessing of the data with use of the Laplacian filter 610. Theaddition of the two values by the adder 608 causes the value at the edgeportion larger than the real value or the edge can be emphasized.

As displayed in Table 13, the SHARP₅₋₀ controls the bank of the MTFcontrol table in the region discrimination section 74, the selection ofthe smoothing filters 602 and 604 in the MTF correction section 78 andthe ON/OFF of the output of the Laplacian filter 610 to respond to thesharpness mode SHARP designated externally.

                                      TABLE 13                                    __________________________________________________________________________                            multiplier                                            sharpness                                                                           SHARP       smoothing                                                                           for                                                   mode  5 4 3 2 1 0 processing                                                                          V.sub.E7-0                                                                          MTF data                                        __________________________________________________________________________    weak  --                                                                              --                                                                              --                                                                              0 0 0 5 × 5                                                                         ×0                                                                            MTF data A                                      ↑                                                                             0 0 0 1 0 0 smoothing                                                                           ×0.25                                                                         (MPX2 = "L")                                          0 0 1 1 0 0       ×0.5                                             standard                                                                           0 1 0 1 0 1 3 × 3                                                       0 1 1 1 0 1 smoothing                                                                           ×0.75                                           ↓                                                                            1 0 0 1 0 1       ×1                                                    1 0 1 1 1 --                                                                              none                                                              1 1 0 1 1 --      ×1.25                                           strong                                                                              1 1 1 1 1 --      ×1.5                                            map mode                                                                            --                                                                              --                                                                              --                                                                              1 1 --      --    MTF data B                                                                    (MPX2 = "H")                                    __________________________________________________________________________

(h-5) color blur correction

The color blur correction is performed by vanishing the data C, M, Y ofchromatic reproduction colors in a black edge portion of an originalimage according to the achromatic edge decision signal EDG generated inthe region discrimination section 74. As explained above, the EDG signaland the read mode signal MODE (photograph/standard) are inputted to theLaplacian table (color blur correction table) 620.

The color blur correction is performed if EDG="L" (achromatic color edgeportion) and MODE="H" (standard mode). The output D is set to be -64irrespective of the input value A. Then, the product with the MTF dataobtained in the following multiplier 622 is forced to be negative.Therefore, if PD₁₇₋₁₀ is C, M, Y data (CMY/K="L"), -64*(MTF data) issubtracted from the PD₁₇₋₁₀, and the C, M, Y data are vanished in theblack portion to prevent color blur.

Next, examples of the color blur correction is explained. If theabove-mentioned automatic UCR/BP processing is performed on the readdensity data of G, B, R shown in FIG. 59 in the color correction section72, the output results shown in FIG. 60 can be obtained. (In FIG. 60,the UCR/BP ratios are set to be about 80%.) The abscissa designates thepixel number. Color blur phenomena arise in the edge portions designatedby open circles shown in FIG. 60. That is, at the edge portion of K, thedata C, M and Y also have edge portions. Therefore, chromatic colorsoverlay besides black, so that colors bleed and the edge cannot bereproduced clearly. As shown in FIG. 61, if the edge emphasis isperformed in the MTF correction in the MTF correction section 78 withoutthe color blur correction, the color blue is exaggerated further (referthe portions designated by circles). On the other hand, as shown in FIG.62, if the color blur correction is performed in the achromatic edgeportion in the standard mode according to the EDG and MODE signals, theoutputs of C, M, Y at the black edge portions vanish, and the color bluris solved as shown in the portiones designated by circles.

(h-6) contour extraction mode

The edge detection signal RAP sent from the Laplacian filter 610 (FIG.53) is compared with the threshold value REF in the edge level decisioncircuit 630 to generate the contour extraction signal RAP. That is, theedge detection signal RAP is first converted to the absolute value ifnegative and to "00" if positive. Next, the resultant value is comparedwith the threshold value REF to convert to a bi-level signal, and if thevalue is larger than the threshold value, RAP="L" is sent to theselector 632 (refer FIG. 63). The selector 632 receives the image dataand EDG data, and if RAP="L" and EGEN2 (contour extraction signal)="L",the EDG data (contour data) is selected.

FIG. 63 shows an example of the contour extraction. As to the densityconversion data of a document image of a large character "A" shown atthe top of FIG. 63, the second derivative is calculated, and if thesecond derivative is negative, the second derivative data is convertedto an absolute value while if positive, to "00". The RAP signal isgenerated only for the converted value larger than the threshold valueREF, and this shows the contour portion.

The REF₁₇₋₁₀ (binarization level for contour extraction), the EDG data(contour data) and the MTF data B (edge emphasis data) are set in theaddress map of the CPU, as shown in Table 14. The data are set at therising edge of the WR signal when GCS1="L".

                  TABLE 14                                                        ______________________________________                                        MA 1.0          content of register                                           ______________________________________                                        0               set MTF data B in MD.sub.7-0                                  1               set REF .sub.17-10 in MD.sub.7-0                              2               set EDG data in MD.sub.7-0                                    3               --                                                            ______________________________________                                    

Further, as shown in Table 15, two MTF data are divided: If MPX2="L",the MTF data is selected to be the MTF data A of the regiondiscrimination section 74 used for the automatic control of edgeemphasis (photograph/standard mode), while the MTF data is selected tobe the MTF data B used for the manual control according to the set value(MTF data B) in the register (map mode).

                  TABLE 15                                                        ______________________________________                                        MPX2      MTF data (output of selector 644)                                   ______________________________________                                        "L"       MTF data A (output of MTF control table                                       in region discriminator)                                            "H"       MTF data B (value to be set in register)                            ______________________________________                                    

Further, Table 16 shows the bit definition of the MTF data B.

                  TABLE 16                                                        ______________________________________                                        bit  9     8     7   6    5    4    3    2    1    0                          ______________________________________                                        digit                                                                              SIN   2     1   1/2  1/4  1/8  1/16 1/32 1/64 1/128                      ______________________________________                                    

In the map mode, the priority is given to the reproducibility of narrowline, the parameters are set so that MTF data B="80" (1), MODE="H"(standard mode) and BKEN="H" (without color blue correction).

(h-7) examples of automatic MTF correction

FIGS. 64-66 shows an example of the automatic MTF correction of adocument of 1 line/mm. FIG. 64 shows the value data and its primaryderivative, wherein the abscissa represents the pixel position. It isfound that the value changes according to each line. FIG. 64 shows thedensity data and its secondary derivative.

FIG. 66 shows the result of the automatic MTF correction for the densitydata. The processing similar to the edge emphasis is performed. It isbetter for an image of line pairs to have a sharper boundary. Therefore,the reproducibility of the image is improved.

For comparison, FIG. 67 shows the result of the smoothing processing. Itis found that the reproducibility of line pairs is not good. FIG. 68shows the result of the usual edge emphasis, and this shows a similardata as the present embodiment.

FIGS. 69 and 70 show an example of the automatic MTF correction for thedensity data of a dot document (screen 133 line). FIG. 69 shows thedensity data, wherein the abscissa designates the pixel position. Themoire pattern is observed in the data.

FIG. 70 shows the result of the automatic MTF correction for the densitydata. It is found that the moire pattern is not emphasized.

For comparison, FIG. 71 shows the result of the edge emphasis, whereinthe moire pattern is emphasized. Further, FIG. 72 shows the result ofthe smoothing processing. It is found that the density change becomessmaller.

FIGS. 73-75 show an example of the automatic MTF correction for adocument of a black line of 1 mm width. FIG. 73 shows the value data andits primary derivative. It is found that the value changes in responseto the black line. Further, FIG. 74 shows the density data and itssecondary derivative.

FIG. 75 shows the result of the automatic MTF correction. It is foundthat the smoothing effect is large at the flat portion of the image,while the gradient of the edge becomes large at the edge portions of theimage. Therefore, the reproducibility of the image of black line isgood.

For comparison, FIG. 76 shows the result of the smoothing, wherein thegradient of the edge becomes gradual. Further, FIG. 77 shows the resultof the edge emphasis. It is found that the density changes largely atthe flat portion of the image.

As explained above, the optimum MTF correction can be performed for anykind of image by using the automatic MTF correction of the presentinvention.

Though the above-mentioned MTF correction is applied to the full colormode, a similar MTF correction can be applied to the monochromatic colormode. In this case, a circuit as shown in FIG. 25 is used. In themonochromatic color mode, the read data can be treated as the valuedata. Therefore, the construction of the circuit can be simplified.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

What is claimed is:
 1. An image processor, wherein each image data isprocessed to obtain image data for printing for each pixel, comprising:aread means for reading a document image to send an image data of threeprimary colors of an additive color system; an edge decision means fordeciding an edge image portion according to the image data received fromsaid read means; an achromatic color decision means for deciding anachromatic color according to the image data received from said readmeans; a generation means for generating image data of three primarycolors in a subtractive color system and of a black color from the imagedata received from said read means; a correction means for conductingedge emphasis processing not on the image data of the three primarycolors in the subtractive color system, but on the image data of theblack color, in an edge image portion of achromatic color decided bysaid edge decision means and said achromatic color decision means.
 2. Animage processor according to claim 1, further comprising a first signalgeneration means for generating a value signal from the image dataobtained by said read means, wherein said edge decision means decidesthe edge image portion according the value signal.
 3. An image processoraccording to claim 1, further comprising a second signal generationmeans for generating two kinds of color difference signals from theimage data obtained by said read means, wherein said achromatic colordecision means decides an achromatic color image portion according tothe two kinds of color difference signals.
 4. An image processor,wherein each image data is processed to obtain image data for printingfor each pixel, comprising:a read means for reading a document image tosend an image data of three primary colors of an additive color system;a calculation means for calculating the chroma from the image datareceived from said read means; a generation means for generating imagedata of three primary colors in a subtractive color system and of ablack color from the image data received from said read means; a firstdetermination means for determining an under color remove amount whichis subtracted from the image data of the three primary color generatedby said generation means according to the chroma calculated by saidcalculation means; and a second determination means for determining ablack paint image data to be used as one of image data for printingaccording to the chroma calculated by said calculation means.
 5. Animage processor according to claim 4, wherein said first determinationmeans increases the under color remove amount with increasing chroma. 6.An image processor according to claim 4, wherein said seconddetermination means increases the black paint image data with increasingchroma.
 7. An image processor, wherein each image data is processed toobtain image data for printing for each pixel, comprising:a read meansfor reading a document image to send an image data; a first generationmeans for generating a value signal from the image data obtained by saidread means; a first edge detection means for detecting a change in thevalue signal generated by said first generation means; deciding an edgeimage portion according to the image data received from said read means;a second generation means for generating image data for printing fromthe image data obtained by said read means; a second edge detectionmeans for detecting a change in the image data for printing generated bysaid second generation means; a synthesis means for synthesizing thechange of the value signal detected by said first edge detection meansand the change of the image data detected by said second edge detectionmeans as to the same pixel in the document image; and a correction meansfor conducting edge emphasis processing on the image data for printingaccording to the change synthesized by said synthesis means.
 8. An imageprocessor according to claim 7, wherein said image data for a printingis image data for black color.
 9. An image processor, wherein each imagedata is processed to obtain image data for color reproduction for eachpixel, comprising:a read means for reading a document image to send animage data of three primary colors of an additive color system; a firstgeneration means for generating a value signal from the image dataobtained by said read means; a first edge detection means for detectinga change in the value signal generated by said first generation means;deciding an edge image portion according to the image data received fromsaid read means; a second generation means for generating image data forcolor reproduction from the image data obtained by said read means; asecond edge detection means for detecting a change in the image data forcolor reproduction generated by said second generation means; asynthesis means for synthesizing the change of the value signal detectedby said first edge detection means and the change of the image datadetected by said second edge detection means as to the same pixel in thedocument image; and a correction means for conducting edge emphasisprocessing on the image data for color reproduction according to thechange synthesized by said synthesis means.
 10. An image processoraccording to claim 9, wherein said synthesis means comprises means forobtaining a product of the change of the value signal obtained by saidfirst edge detection means with the change of the image data for colorreproduction obtained by said second edge detection means.
 11. An imageprocessor according to claim 9, wherein said correction means conductsthe edge emphasis processing on a plurality of the image data for colorreproduction successively.
 12. An image processor according to claim 11,wherein said plurality of image data includes image data of threeprimary colors in a subtractive color system.